In the rubber industry, there is a general demand for better processability of the rubbers employed. This concerns the flow properties in particular. The lower the viscosity of the crude rubber, the simpler the processing technology, the greater the productivity and the lower the waste. The aspects mentioned are of great relevance to fluorinated rubbers in particular, since these are expensive rubbers which cannot be processed without reservation on the injection moulding machines of the rubber industry.
The process of emulsion polymerization which has hitherto been customary on an industrial scale is unsuitable for the preparation of low-viscosity fluorinated rubbers, such has have been described e.g. in DE-A 196 40 972.1, since on the one hand the need to use ionic inorganic initiators leads to ionic end groups in the polymer, which increase the viscosity of the polymer, and on the other hand large amounts of molecular weight regulators are necessary, and these as a rule lead to a drastic reduction in the reactor productivity because of side reactions with the initiator in the aqueous phase and termination reactions.
Of the non-aqueous processes, polymerizations in the pure liquefied fluorine monomer have proved to be unfavourable, since the polymers formed are usually not soluble therein and they possess poor swelling properties. A reproducible polymerization procedure with good transfer of heat and material and therefore acceptable reactor productivity is possible to just as little an extent by this route.
In contrast, fluorine monomers can be readily polymerized in the presence of certain fluorine-containing solvents, see e.g. U.S. Pat. No. 4,243,770, DE-A 196 40 972.1. U.S. Pat. No. 5,182,342 describes the use of fluorinated hydrocarbons as the polymerization medium, which meet certain criteria in respect of the F/H ratio and the position of the hydrogens. With all compounds of this type which contain hydrogen and optionally also additionally chlorine, there is always the problem that these can undergo transfer and/or termination reactions. In the case where defined end groups are to be introduced by the molecular weight regulator, however, no additional transfer reactions should start from the solvent under the reaction conditions chosen. This applies in particular to the preparation of low-viscosity rubbers which contain covalently bonded iodine on the terminal carbon atoms due to the use of diiodo-organic chain transfer agents. In DE-A 196 40 972.1, this is achieved by the use of 1,1,2-trichlorotrifluoroethane as the polymerization medium. Compounds of this type (fluorochlorocarbons), however, have a considerable ozone-depleting potential. For this reason, their use in industry is already banned in many industrial countries.
The object of the present invention was therefore to discover a practicable process for the preparation of low-viscosity fluorinated rubbers which proceeds without the use of ozone-depleting compounds.